Clean up CSS border color styles

Created by: sqs

Previously, we had many ways of setting border colors:

• Sometimes the hex color values would be used directly, and a component would define its own light and dark theme variants. This leads to inconsistent color usage, adds code complexity, and is mistake-prone.
• We had $color-border, $color-border-2, $border-color, --border-color, etc.

Now we just have --border-color and --border-color-2 (dimmer, rarely used). Some components' borders will have slightly different colors after this change if they previously used a non-standard border color.